1. Field of the Invention
The invention relates generally to systems and methods for lifting the rotor of a downhole progressive cavity pump. More particularly, the invention relates to systems and methods for pulling the rotor of a downhole progressive cavity pump while retarding the backspin of the rod string coupled to the rotor.
2. Background of the Invention
Progressive cavity pumps, also known as “Moineau” pumps, pump a fluid via a sequence of small, discrete, sealed cavities that progress from one end of the pump to the other. Progressive cavity pumps are commonly used in oil and gas development operations. For instance, progressive cavity pumps may be used to produce a low pressure oil well or to raise water from a borehole.
As shown in FIGS. 1 and 2, a conventional progressive cavity pump 10 includes a helical-shaped rotor 30, typically made of steel that may be chrome-plated or coated for wear and corrosion resistance, disposed within a mating stator 20, typically a heat-treated steel tube 25 lined with a helical-shaped elastomeric insert 21. Rotor 30 defines a set of rotor lobes 37 that intermesh and periodically seal with a set of stator lobes 27 defined by insert 21. As best shown in FIG. 2, rotor 30 typically has one fewer lobe 37 than stator 20. When rotor 30 and stator 20 are assembled, a series of cavities 40 are formed between the outer surface 33 of rotor 30 and the inner surface 23 of stator 20. Each cavity 40 is sealed from adjacent cavities 40 by seals formed along the contact lines between rotor 30 and stator 20. As best shown in FIG. 2, the central axis 38 of rotor 30 is offset from the central axis 28 of stator 20 by a fixed value known as the “eccentricity” of the rotor-stator assembly.
Stator 20 is traditionally suspended on a string of tubing which hangs inside the well casing, and rotor 30 is typically disposed on the downhole end of a rod string (not shown). At the surface, a drivehead or motor transmits rotational motion to rotor 30 through the rod string. Depending on the length of the rod string, the upper end of the rod string coupled to the drivehead may rotate ten to 20 turns before downhole rotor 30 begins to rotate, resulting in significant torsional energy build-up in the rod string. As rotor 30 is rotated relative to stator 20, fluid contained in cavities 40 between rotor 30 and stator 20 is pumped toward the surface via the sequence of discrete cavities 40 that move through pump 10. As this rotation and movement of cavities 40 repeats in a continuous manner, the fluid is transferred progressively along the length of pump 10. The volumetric flow rate of fluid pumped by pump 10 is generally proportional to the rotational speed of rotor 30 within stator 20. In addition, the fluid pumped in this manner experiences relatively low levels of shearing, which may be important for transferring viscous or shear sensitive fluids.
On occasion, the rotor of a progressive cavity pump (e.g., rotor 30) may need to be pulled or lifted from its mating stator (e.g., stator 20) for maintenance, repairs, or to free a rotor that gets stuck or jammed within the stator. For instance, a rotor pumping a fluid with a high water and sand content may get stuck if the pump does not provide sufficient velocity to carry the sand to the surface. In such a well, the sand may settle out on top of the pump. The sand may continue to settle out on top of the pump until it creates a sufficient flow restriction to overcome the power of the surface drivehead. As another example, a rotor may become stuck in the stator because of an incompatible fluid. Some fluids passing through a progressive cavity pump may interact with the stator (e.g., elastomeric stator) and cause the stator to swell or contract. If the stator swells sufficiently, it may over-engage the rotor resulting in frictional force sufficient to overcome the power of the drivehead.
When the rotor becomes stuck, the rotor can no longer rotate within the stator. As a result, the downhole progressive cavity pump is unable to pump fluid, and further, the drivehead at the surface may stall. In such cases, it may be necessary to pull the rotor from the stator. However, when the upper end of the rod string is disengaged from the drivehead to pull the rotor, there is a tendency for the rotor and rod string to “backspin.” The tendency to backspin results from the combination of two factors. First, the rod string functions like a powerful torsion spring when it is decoupled from the drivehead—the build-up of torsional energy in the rod string resulting from the twisting referred to above tends to rotate the rod string backwards. Second, when the rotor is pulled from the stator, the column of fluid (i.e., fluid head) above the progressive cavity pump will tend to flow back down under the force of gravity past the pulled rotor and through the stator. As the fluid flows past the rotor it tends to cause the helical-shaped rotor to function like a progressive cavity motor and rotate backwards. In some cases, the backspin of the rod string experienced when the rotor is pulled may exceed 1000 RPM.
The acceleration and rotational velocity of a back-spinning rod string presents a variety of potential safety hazards at the surface. For instance, the upper end of the rod string, also referred to as a “polish rod”, may bend over while back-spinning, potentially impacting nearby persons or objects. In addition, a bent polish rod may send debris flying across the worksite. Further, extreme vibrations generated by the violent back-spinning may cause weaken or damage the support structure surrounding the rod string at the surface. Moreover, in some cases, contact between metal parts with high relative rotational velocities may result in sparks that could ignite combustible gases and hydrocarbon liquids at the surface.
Accordingly, there remains a need in the art for devices, methods, and systems to more safely lift a rotor from a downhole progressive cavity pump. Such devices, methods, and systems would be particularly well received if capable of retarding the backspin of the rod string employed to pull the rotor.